Refactor automatic schema fetching
Refactor automatic schema fetching

I removed `operation_and_maybe_refetch_schema` in favour of
`refetching_schema` in the `MarcoPolo` module. This new function accepts
a `conn` (on which it will call `fetch_schema/1`) and a function whose
return value determines whether the schema is going to be refetched.

MyMedsAndMe/marco_polo,linearregression/marco_polo